1. Technical Field
Example embodiments relate generally to a nonvolatile memory device. More particularly, embodiments of the present inventive concept relate to a method of adjusting read voltages for a NAND flash memory device.
2. Description of the Related Art
A NAND flash memory device can store data by injecting an electric charge into a floating gate and read the stored data at a time when requested. The NAND flash memory device includes a plurality of memory cells. Each memory cell stores one-bit data or multi-bit data. In the single-level cell (SLC) NAND flash memory device, each memory cell corresponds to one of two threshold voltage states to store one-bit data. In contrast, in the multi-level cell (MLC) NAND flash memory device, each memory cell corresponds to one of four threshold voltage states to store 2-bit data. Further, in the triple-level cell (TLC) NAND flash memory device, each memory cell corresponds to one of eight threshold voltage states to store 3-bit data.
Therefore, the MLC NAND flash memory device can store relatively large amounts of data because the memory cell of the MLC NAND flash memory device stores more data than the memory cell of the SEC NAND flash memory device. Generally, the MLC NAND flash memory device can reduce the chip size to about 35% and the manufacturing cost to about 40% in comparison with the SLC NAND flash memory device.
On the other hand, the MLC NAND flash memory device has a relatively low read/write/erase performance in comparison with the SLC NAND flash memory device because each memory cell of the MLC NAND flash memory device stores multi-bit data. Generally, a read speed of the MLC NAND flash memory device is about 30% lower than a read speed of the SLC NAND flash memory device, and a write speed of the MLC NAND flash memory device is about 75% lower than a write speed of the SLC NAND flash memory device. In addition, an erase speed of the MLC NAND flash memory device is only about 1/10 of an erase speed of the SLC NAND flash memory device. Overall, the performance of the MLC NAND flash memory device is about 60% lower than the performance of the SLC NAND flash memory device. In spite of these disadvantages, the MLC NAND flash memory device is widely used because of the advantage of low cost.
Recently, in the NAND flash memory device each of which memory cell stores multi-bit data for storing relatively large amounts of data. methods of reducing the read error are developed to improve the reliability the of the read data. For example, because a threshold voltage distribution is gradually broadened due to charge loss and charge acquisition, the read error occurs by a variation of the threshold voltage distribution in the MLC NAND flash memory device. Therefore, it is necessary to adjust the read voltages according to the variation of the threshold voltage distribution.